The present invention relates generally to stents which are implantable or deployable in a vascular or endoluminal location within the body of a patient to maintain the lumen open at that site, and more particularly to improvements in stent flexibility, particularly longitudinally.
Stents are expandable prostheses employed to maintain narrow vascular and endoluminal ducts or tracts of the human body open and unoccluded, such as a portion of the lumen of a coronary artery after dilatation of the artery by balloon angioplasty. While vascular usage is frequently discussed in this application, it will be understood by those skilled in the art that stents having the characteristics and features of the present invention may be implanted in other ducts or tracts of the human body to keep the lumen open, such as in the tracheobronchial system, the billiary hepatic system, the esophageal bowel system, and the urinary tract system.
In the case of an occluded coronary artery, for example, the original blockage is typically attributable to fat deposits or plaque on the inner lining of the vessel. A new blockage often occurs after an angioplasty procedure is performed to compress the deposits against the inner lining of the vessel, as by use of balloon angioplasty, or to virtually entirely remove the deposits, as by use of laser angioplasty or rotational cutting. The blood vessel wall is subjected to trauma by such procedures, leading to neointimal hyperplasia, i.e., rapid cellular proliferation in the affected region of the wall, and thereby causing restenosis and re-occlusion of the vessel lumen in a significant percentage of angioplasty patients within a period of from three to six month s following the initial procedure.
To avoid this re-occlusion and to maintain the lumen of the vessel open, it is customary procedure to install a stent at the angioplasty site in the vessel. The stent is deployed by radial expansion of its wall as pressure is exerted by controlled inflation of the balloon of a balloon catheter on which the stent is mounted. In this way, the stent wall is caused to engage the inner lining or surface of the vessel wall with sufficient resilience to allow some contraction and, desirably, with sufficient stiffness to resist or minimize the natural recoil of the vessel wall. Recoil is the reaction of the vessel wall to an even slight expansion of its diameter when the stent is deployed, owing to the elastic retraction force of the vessel wall. Recoil produces a re-narrowing of the vessel after the stent is implanted compared to the vessel diameter when the balloon is inflated.
The stent provides not only the benefits of reducing restenosis following vascular intervention such as a coronary angioplasty, but also reduces acute complications such as acute vessel closure. Widespread use of stents has demonstrated their benefit in applications beyond merely coronary implantations, such as in iliac, femoral, infragenouidal, carotid and other vascular applications. Additionally, stents have been found to be important in treating other vessels and ducts, such as biliary, esophageal and tracheal applications, to mention a few. In these applications also, the primary purpose of the stent is to keep open a lumen that might otherwise become occluded by a neoplasia.
Nevertheless, some limitations remain in current methods of use of stents. Although the extent of restenosis of the vessel is reduced, its remaining impact is of sufficient magnitude to represent a serious medical and economic problem. A principal part of the problem is attributable to individual patient-related factors, such as vessel size, diabetes, degree of stenosis prior to the intervention, and the type, length and morphology of the lesion (i.e., the region of narrowing that prompted the intervention). The problem is also attributable in significant part to stent-related factors of a mechanical nature. These include force distribution of the implanted stent on the vessel wall, symmetry of opening of the stent, metal surface and geometric design creating sharp edges and corners.
Some current stent designs have sought to provide high mechanical stability for resisting recoil after stent implantation. The two counter-acting forces, one being the elastic recoil exhibited by the highly overexpanded vessel wall and the other being the radial strength of the expanded stent, are being brought to a state of balance. Stent designs of the slotted tube and multicellular types have provided suitable mechanical stability. Slotted tube stent designs utilize a plurality of slots which are disposed substantially parallel to the longitudinal axis of the tubular member. Depending on the length of the lesion at the site to be treated, it may be necessary to implant more than one stent in longitudinal alignment. To achieve greater longitudinal flexibility, adjacent stents may be connected by connecting members of the type described in European Patent Application No. 89118069.7 of R. Schatz for Expandable Intraluminal Graft.
U.S. Pat. No. 5,304,200 to R. Spaulding describes a stent with a plurality of adjacent generally circumferential sections that are substantially axially positioned with respect to each other. The terminal portions of the end circumferential sections are welded directly to a portion of a generally adjacent circumferential following section.
European Patent Application No. 92309822.2 to L. Lau et al describes an expandable stent with circumferential bending rings interconnected to each other by connecting members, which are offset from face to face.
U.S. Pat. No. 5,824,045 to E. Alt describes a slotted tube stent in which circumferential sinusoidal rings are interconnected to each other, and a gold coating is applied to enhance visibility and reduce thrombogenicity.
U.S. Pat. No. 5,827,321 to G. Roubin et al describes an intraluminal prosthesis in which a plurality of connecting members connect the a pieces of adjacent annular members, the connecting members having a plurality of alternating segments that function to compensate for the smaller longitudinal dimension of each annular member in the expanded state.
U.S. Pat. No. 5,843,117 to E. Alt describes a stent with serpentines that are substantially devoid of sharp corners and edges, where each serpentine has an oval cross-section, and adjacent serpentines are joined together at crest and trough respectively so that their interconnections are 180.degree. out of phase relative to their wavelength.
In addition to the type of multicellular or tubular stent that provides high mechanical strength and low recoil, a second type of stent known as a coil stent has been described in the prior art.
U.S. Pat. No. 4,886,062 to D. Wiktor describes a vascular stent comprising a cylindrical open-ended wire component in a zig-zag pattern to allow for radial expansion without significantly shortening its length, and in which a single filament continuous from one end to the other forms the support structure. Such a stent has an advantage of increased longitudinal flexibility, but at a sacrifice of radial support owing to absence of an annular structure.
A similar type of stent is described in U.S. Pat. No. 5,370,683 to A. Fontaine, in which the stent is formed from a single filament of low memory biocompatible material having a series of U-shaped bends, the filament being wrapped around the mandril in circumferential fashion so that the curved portions of each bend are aligned. This stent similarly retains high flexibility but suffers from lack of a ring structure to increase support and reduce recoil.
U.S. Pat. No. 5,591,230 to J. Horn also describes a stent fabricated from a single filament wire, where the wire forms an original multi-loop design including a plurality of concentric bended loops in a continuous wire folded along a length thereof Like the two immediately preceding patented designs, this stent design provides longitudinal flexibility but less radial support.
Although the best clinical results appear thus far to have been achieved with the multicellular designs, and coil stents have been found to achieve less favorable long-term outcome after implantation in the patient's coronary system from the standpoints of restenosis and complication rate, none of the current stent designs seem to pay sufficient attention to the biomechanics of the native human coronary vessel. A rigid tubular stent undergoes little mechanical bending longitudinally when implanted in the coronary ostium, but encounters a problem when implanted for treatment of a more distal coronary lesion because of increased longitudinal bending of the vessel. The native coronary vessels in patients with hypertension, in particular, exhibit increased bending with considerable changes in the radius of the bend following systole and diastole. The coronary vessel flexes more than 100,000 times a day following myocardial contraction--more than 400 million in ten years. Implantation of a relatively rigid stent, or any stent with severely limited longitudinal flexibility, in this region of increased mechanical stress creates a substantial problem because the bending is not equally distributed over the entire length of the stent. Rather, bending is primarily limited to two major points at the proximal and distal ends of the stent, in the transition between the edges of the stent and the vessel. The increased mechanical stress in this region represents an increased risk for restenosis, especially when bifurcations are also involved.
A representative example of a prior art stent is illustrated in FIG. 1. Spaced-apart, circumferentially disposed, identical sinusoidal ring structures 10 are stacked longitudinally and have their respective crests 12 and troughs 13 aligned longitudinally. Adjacent pairs of individual rings such as 11, 14 and 17 are interconnected crest-to-crest or trough-to-trough by straight longitudinal elements such as 15, 16 and 18, 19, in which the interconnecting elements 15, 16 are offset from interconnecting elements 18, 19 to allow more longitudinal flexibility of the overall structure. Some of the interconnecting elements may be bent, rather than straight, such as the elements 21, 22 connecting rings 17 and 20. Although this prior art design allows some flexibility when implanted, the compliance of the expanded stent is considerably less than the compliance of the natural vessel in which it is implanted, particularly in the aforementioned regions of increased mechanical stress.
A significant problem with stent designs exemplified by that of FIG. 1 is that improvement in longitudinal flexibility requires that the interconnecting elements 15, 16, 18, 19, 21, 22 be made very thin. Since the coronary vessels as well as other vessels in the cardiovascular system undergo longitudinal bending at the rates mentioned above, it is clear that mechanical limitations impose significant barriers to making the interconnecting elements very thin and flexible.
Therefore, the principal aim of the present invention is to provide a stent design that offers considerable mechanical support against inwardly directed radial forces as occur with vessel recoil, and gives excellent coverage of the narrowed vessel region to diminish local wall stress, but which also allows optimum longitudinal flexibility of the vessel to avoid compromising the natural bending of the vessel and the implanted stent that occurs with systolic and diastolic contractions and relaxations of the heart.
A further aim of the invention is to maintain the mechanical integrity and stability of such a stent in the process of fabricating the stent and mounting it on a balloon or other means by which the stent is to be implanted in the patient's body, but to release or surrender this mechanical stability when the stent is implanted and deployed at the preselected site in the vessel, duct, tract or orifice of the body at which the stent is designed to perform its primary function (i.e., the target site).